Update to readable.css v1.1.0 (#13)

* Update to readable.css v1.1.0

data-style="classy" was also added to the <nav> element to preserve the
same style as the previous default.

* Use minified version of readable.css to save bandwidth

The readable.css file is kept for readability sake of people using this
template.

---------

Co-authored-by: Benjamin Hollon <me@benjaminhollon.com>

layouts/partials/header.html

@@ -1,10 +1,10 @@
 <h1>{{ $.Site.Title }}</h1>
 <p>{{ $.Site.Params.subtitle }}</p>
 
-<nav>
+<nav data-style="classy">
   {{ range .Site.Menus.main -}}
   <span>
       <a href="{{ .URL | absLangURL }}">{{ .Name }}</a>
   </span>
   {{- end }}
-</nav>
+</nav>